Must Vertex Group Edits Remove Current Before Assigning New?
As it turns out it seems it is just bad wording of how the controls are labeled. When it says "remove" for a vertex group it does mean that. When it says "assign" it is misleading...it should instead say "add" or "insert".

I do wish there were another button "assign" which would set the vertex group state to the current selected set of vertices...removing ones previously selected but not currently selected.